As a machine tool for beautifully polishing up the surface of round bar metals and other metal products, there has been in use a machine commonly known as the grinder. Since, however, its function is to grind off the surface of a workpiece which is pressed against a rotating grinding wheel to polish it up, it is primarily employed for the purpose of removing flaws on the surface of a workpiece or of reducing products to correct dimensions and it was impossible to beautifully polish up the surface of round bar metals drawn to a specified diameter, that is, to give it a good shine without changing the diameter.